native: validate the Linux AppImage release and updater end to end

[tlgimenes]

Context

#5556 merged the x86_64 Linux AppImage build, updater channel, installer, download surface, FUSE org filesystem, and Linux CI. Passing CI proves buildability and headless boot, but not the complete production path on a real desktop.

The first merge-triggered release is https://github.com/decocms/studio/actions/runs/32299482446.

Acceptance checklist

• Verify the published release contains the versioned Linux x86_64 AppImage, checksum/signature material, and a Linux entry in latest.json.
• Install through the production web/download and install.sh paths, then launch the installed AppImage.
• Smoke Ubuntu 22.04 and 24.04 on a real graphical session; record desktop environment, WebKitGTK version, and required host packages.
• Verify sign-in and persistence with an available desktop credential service.
• Verify the keychain-unavailable recovery UI when no credential service is available.
• Start and stop a sandbox, load its preview, and verify restart/cleanup.
• Verify org/ over FUSE with fusermount3 present.
• Verify the missing-FUSE-helper failure is bounded and actionable.
• Publish a subsequent Linux release and prove an installed N updates to N+1, including signature verification, staged install, quit/restart, and version reporting.
• Record the tested support matrix and link it from the Linux documentation issue.

Exit criterion

Linux can be called beta-supported when first-install and real-desktop smoke pass. It can be called updater-supported only after a real N -> N+1 release succeeds.

[tlgimenes]

Linux release-channel validation is complete for the first published artifact:

• Immutable release: https://github.com/decocms/studio/releases/tag/native-v4.242.0
• Successful build/publish workflow: https://github.com/decocms/studio/actions/runs/32301701283
• Published AppImage: deco-4.242.0-linux-x86_64.AppImage
• Published SHA-256: d6c6fa4da48b1985af027b258d3d2a00bb101d1cbf076a9fcc3ade9f1c84c8d2
• The public rolling manifest is now 4.242.0 and contains signed darwin-aarch64 and linux-x86_64 entries.
• The workflow's AppImage tooling, runtime/libfuse, updater signature, and updater-platform coverage checks passed.

This clears artifact publication and updater discovery. Real Ubuntu desktop smoke testing and a Linux N-to-N+1 updater test remain open before GA.

Related hardening: #6282
Related documentation: #6283
Promotion-throttle regression: #6288